Researchers at the ALS have demonstrated a new method to generate tunable, coherent, broadband terahertz radiation from a relativistic electron beam modulated by a femtosecond laser. Interaction of the ALS electron beam with a femtosecond laser pulse as they co-propagate through a wiggler modulates the electron energies within a short slice of the electron bunch with about the same duration as the laser pulse. This causes a dispersion of the electron trajectories, and the bunch develops a hole that emits short pulses of temporally and spatially coherent terahertz pulses synchronized to the laser. The technique allows tremendous flexibility in shaping the terahertz pulses by appropriate modulation of the laser pulse.

The electromagnetic spectrum. The scarcity of intense broadband sources of radiation in the 1012 hertz (terahertz) frequency range leaves us blind to a wide range of interesting phenomena.

Filling the Terahertz Gap

To "see" phenomena invisible to the naked eye—from galaxies to viruses and atoms—scientists have the entire electromagnetic spectrum at their disposal: radio waves, microwaves, and infrared light below the visible spectrum, and ultraviolet light, x rays, and gamma rays above it. However, between the microwave and infrared regions, where electromagnetic waves oscillate at a frequency of 1012 hertz (terahertz), we have a noticeable blind spot. An almost limitless array of interesting phenomena occur at terahertz frequencies: Excited electrons orbit, small molecules rotate, proteins vibrate, superconducting energy gaps resonate, and gaseous and solid-state plasmas oscillate. The list goes on.

The reason for this "terahertz gap" has been the scarcity of intense broadband sources of radiation in the terahertz frequency range. Several desirable features of such a source include pulses shorter than 100 femtoseconds, synchronization to another ultrafast source ranging from infrared to x-ray wavelengths, and the ability to shape the time envelope of the pulse. Among the promising technologies under study is the relatively new development of coherent synchrotron radiation generated by electron storage rings. Byrd et al. have demonstrated a new method to generate tunable coherent terahertz radiation with tremendous pulse-shape flexibility by modulation of the laser pulse.

The earlier demonstration of femtoslicing at the ALS as a source of ultrafast x rays showed that we can manipulate the distribution of an electron beam using a short-pulse laser on a time scale of several hundred femtoseconds. One of the by-products of this technique is that femtoslicing can create very short "holes" in the time distribution of the electron bunch. While short electron bunches can radiate coherently (i.e., photons are emitted in phase), the researchers found that these "holes" in the electron bunch can radiate coherently as well, and that this technique could be extended to create a novel source of terahertz radiation. For example, by shaping the slicing laser pulse, we can tailor the shape of the hole that is "sliced" in the bunch and thus shape the electric field of the coherent terahertz pulse. This would make the terahertz radiation tunable.

The effect of a co-propagating laser pulse on an electron bunch. The electron energy distribution (blue band) shows that electrons gaining energy (deltaE > 0) gather toward the back of the bunch (because they follow a longer path), while those that lose energy (deltaE < 0) gather toward the front (because they follow a shorter path). The relative peak bunch current (superimposed white curve) shows a "hole" in the electron bunch that emits coherent terahertz radiation (frequency spectrum shown in inset). As the bunch travels around the storage ring, the "hole" quickly spreads and fills with electrons. Click on the image above to view a movie of the process.

Femtoslicing works by modulating the energy of electrons in the bunch using a high-power laser pulse co-propagating with the electrons in a wiggler field. For example, the interaction of a 75-femtosecond laser pulse with an electron bunch results in the formation of "wings" in the bunch energy distribution. The projection of the distribution on the time axis represents the relative variation in the peak bunch current. As the bunch passes through the accelerator, the high- and low-energy "wings" of the bunch energy distribution slip backward and forward along the bunch, creating a "hole" in the center of the bunch that emits terahertz radiation. As the bunch continues around the storage ring, the "hole" quickly spreads and fills with electrons. Because of the short laser pulses that are used to slice the electron beam, the emission spectrum initially extends up to terahertz frequencies but shifts to lower frequencies as the hole spreads.

To observe these effects, the researchers used a liquid-helium-cooled bolometer sensitive to terahertz wavelengths and recorded bursts of coherent signal coincident with the slicing, which occurred at a 1-kHz repetition rate. They also measured the spectrum of the coherent radiation at Beamline 5.3.1, immediately following the slicing, and at Beamline 1.4, three-fourths of the way around the ring from the slicing. Spectral measurements are difficult at the ALS because the vacuum chamber design has very poor transmission of long-wavelength radiation. Because the coherent signal is very sensitive to the width and depth of the hole, it is currently used as the primary diagnostic signal for optimizing slicing efficiency.

Left: Bolometer signal observed at Beamline 5.3.1 and Beamline 1.4 with slicing on. Right: The spectral measurement of the coherent radiation at the two beamlines.

Given the ability to slice holes in the electron bunch, we can now consider tailoring the terahertz signal to the needs of a particular experiment. Laser technology allows us to shape the temporal profile of the laser pulse and modulate the electron bunch with a multitude of patterns. For example, if we slice the beam with a train of laser pulses, we can apply a periodic modulation on the electron bunch, generating a narrow-band terahertz signal. This signal would be tunable in frequency by varying the laser pulse spacing.